1. Field of the Invention
The present invention relates to a hydraulic pressure sensor failure control system for a belt-type continuously variable transmission.
2. Description of the Prior Art
As a known example of a continuously variable transmission for a vehicle, Japanese laid-open patent publication No. 8-210449 discloses a belt-type continuously variable transmission (hereinafter referred to as a “belt CVT”) with a V-belt.
In this belt CVT, a transmission mechanism is constructed by winding the V-belt around pulleys comprised of a primary pulley connected to an engine side and a secondary pulley connected to an axle side, and groove widths of the primary pulley and the secondary pulley are variably controlled by hydraulic pressure.
A thrust of the pulley is determined according to an input torque and a gear ratio and converted into a hydraulic pressure on the basis of predetermined values such as pressure-receiving areas of the secondary pulley and the primary pulley, and this hydraulic pressure is supplied to the transmission mechanism as a target line pressure.
The primary pulley and the secondary pulley are respectively provided with a first cylinder chamber and a second cylinder chamber, the first cylinder chamber is supplied with a primary pressure obtained by regulating line pressure, and the second cylinder chamber is supplied with line pressure or a secondary pressure obtained by regulating the line pressure. The groove widths of the primary pulley and the secondary pulley are changed by hydraulic pressure supplied to the respective cylinder chambers during running of the vehicle, and gear ratio is continuously changed according to a contact radius ratio (pulley ratio) between the V-belt and the respective pulleys.
In such a belt CVT, when the vehicle is once stopped by removal of a driver's foot from the accelerator pedal and braking during running on a upward slope in a forward “D” range and started again in the “D” range, the vehicle is slightly moved backward by the removal of the driver's foot from a brake pedal, and then a reverse torque is applied to an output shaft of the belt CVT, and a reverse rotation is generated in the pulleys.
Even if the pulley ratio, the input torque, an input rotation speed or the secondary pressure is the same when the pulleys are reversely rotated, balance in hydraulic pressure between the primary pressure and the secondary pressure is lost, the primary pressure is especially reduced by half, and torque capacity of the primary pulley is reduced, so that belt slip may be generated. However, since no control to prevent this disruption of the balance of hydraulic pressure has been conventionally performed, no case where a primary hydraulic pressure sensor fails has been considered.
The above-mentioned reverse rotation of the pulleys is generated also when the vehicle is started again in an “R” range after it is stopped once while running backward in the “R” range on a down slope, whereby the torque capacity of the primary pulley is also reduced.
That is, the reverse rotation of the pulleys discussed herein means a phenomenon that the pulleys are rotated reversely with respect to a direction of a normal rotation of the pulleys (forward direction in the “D” range or backward direction in the “R” range) to be supposed in a currently selected range position. The reverse rotation of the pulleys is used in this meaning hereinafter.